The present invention relates to a composite gradient alloy plate, a manufacturing method thereof and a highly reliable color cathode ray tube which can enhance the material strength (rigidity) of a shadow mask which constitutes a color selection electrode, can improve the etching characteristic and formability, and can decrease the thermal deformation.
With respect to color cathode ray tubes which are used as monitor devices for information equipment or display means of color picture receivers of recent years, a flat face technique which makes a panel (face panel) constituting an image display surface flat has been rapidly spreading. Particularly, when a shadow mask of a press forming type (pressed mask) which has an apertured surface thereof curved in the horizontal direction and in the vertical direction is adopted, the panel of the color cathode ray tube (flat-face tube) having the flat face has an approximately planar outer surface and an inner surface which has a curvature considerably larger than that of the outer surface.
Not only the shadow mask of a color cathode ray tube, but also a large number of plate materials among plate materials for vehicles, air planes and other structures and parts are required to meet a demand for high rigidity. As the above-mentioned shadow mask or plate material, a high rigidity plate material formed of a single plate material or a so-called clad plate material which is formed by mechanically laminating a plurality of metal plates (mainly made of iron or iron alloy) having different physical characteristics has been used conventionally. However, such a plate material per se has a little ability to hold an internal stress which can cope with a temperature change and hence, it has been difficult for the plate material to reliably ensure the shape holding ability by itself when the plate material is made thin.
In selecting the material of the plate body, it has been often a case that the selection is restricted due to material characteristics such as the strength of a product, a plate thickness, formability, stress applying means and the like. Particularly, as one technical task to be solved at the time of designing the above-mentioned flat face tube, the strength of the shadow mask is named. Here, although the explanation will be made using the shadow mask as an example, the same goes for the plate material which is applied to the above-mentioned other products. Although the shadow mask is formed with a curvature which approximates the curvature of the inner surface of the panel, the flat face tube has the small curvature of inner surface of the panel compared with a round face tube which has both of inner and outer surfaces thereof curved and hence, there is no other way but to make the curvature of the shadow mask of the flat face tube also small.
Accordingly, it is difficult to maintain the strength against the partial thermal deformation of an apertured region of the shadow mask or the thermal deformation of the whole shadow mask generated by the elevation of the temperature of the shadow mask upon impingement of electron beams in operation, that is, the strength against a so-called doming phenomenon. Further, it is also difficult for the shadow mask to maintain the physical strength thereof against a fall, a shock and the like.
As the material which ensures the strength of the shadow mask of this type, cobalt-doped Invar material which is produced by doping cobalt in conventional Invar material has been used. Although aluminum killed steel material was used as the shadow mask material (base material), Invar material has been used along with efforts to enhance the definition of the images and to make the screen have a more flattened face.
The cobalt-added Invar material which is used for enhancing the strength of the shadow mask increases the strength by approximately 20% compared with the usual Invar material and can suppress the above-mentioned deformation of the shadow mask. However, the shadow mask produced by doping cobalt in the Invar material has several defects including (1) the cost is pushed up since the cobalt is expensive, (2) the etching efficiency is decreased since the erosion resistance of cobalt is favorable, (3) the workability is decreased and (4) the magnetic characteristics are decreased.
Accordingly, it is an object of the present invention to provide a flat-face type color cathode ray tube having a shadow mask which can decrease drawbacks which such a conventional shadow mask suffers from.
A typical gist of the present invention to achieve the above-mentioned object lies in that a shadow mask material which is served for a color cathode ray tube is made of an iron material which is formed of a plate body of iron alloy having three or more layers which differ in the concentration of an alloy element and has a concentration gradient of the content of the alloy element contained in the plate body which is continuously changed at boundary portions of respective layers of the plate body and in the vicinity of the boundaries. Typical ones out of the constitutions of the present invention are described as follows.
First of all, with respect to a composite gradient alloy plate and a method for manufacturing the plate of the present invention, following constitutions are considered.
(1) In a composite gradient alloy plate having a plurality of constituent layers which are laminated while continuously changing the concentration of an alloy element in an iron-alloy plate containing the alloy element in iron in the thickness direction of the iron-alloy plate,
a tensile stress and a compressive stress which remain in the planer direction of the composite gradient alloy plate at boundary regions of the plurality of constituent layers are directed in opposite directions from each other.
(2) With respect to the constitution (1), the plurality of constituent layers are formed of three layers consisting of one surface layer, the other surface layer and one intermediate layer which is laminated between one surface layer and the other surface layer.
(3) With respect to the constitution (2), the tensile stress remains in one surface layer and the other surface layer, while the compressive stress remains in one intermediate layer.
(4) With respect to the constitution (2), the compressive stress remains in one surface layer and the other surface layer, while the tensile stress remains in one intermediate layer.
(5) With respect to the constitution (2) or (3), the concentration distribution of the alloy element in the planer direction differs from each other among the plurality of constituent layers.
(6) With respect to any one of preceding constitutions (1) to (5), the concentration distribution of the alloy element in the planer direction in one surface layer is approximately equal to the concentration distribution of the alloy element in the planer direction in the other surface layer.
(7) With respect to any one of preceding constitutions (1) to (5), the alloy element is nickel.
(8) In a method for manufacturing a composite gradient alloy plate having a plurality of constituent layers which are laminated while continuously changing the concentration of an alloy element in an iron-alloy plate containing the alloy element in iron in the thickness direction of the iron-alloy plate, the method comprises:
a step in which a plurality of molten materials which differ in the content concentration of the alloy element are merged by hot rolling thus forming a base material for composite gradient alloy plate having a plurality of constituent layers which differ in thermal expansion due to the continuous change of the concentration of the alloy element,
a heating step in which the temperature of the base material for composite gradient alloy plate is elevated from a normal temperature to a temperature at which the constituent layer of high thermal expansion among the plurality of constituent layers is displaced by an amount equal to or more than an elastic limit of the constituent layer of low thermal expansion thus alleviating an internal stress of the constituent layer of low thermal expansion, and
a step in which, after completing the heating, the temperature of the base material for composite gradient alloy plate is made to return to the normal temperature so as to make the constituent layer of low thermal expansion generate a compressive stress and also to make the constituent layer of high thermal expansion generate a tensile stress based on the compressive stress generated in the constituent layer of low thermal expansion,
whereby the internal stress is made to remain in the base material for composite gradient alloy plate.
(9) With respect to the constitution (8), the plurality of constituent layers are formed of three layers consisting of one surface layer, the other surface layer, and one intermediate layer which is laminated between one surface layer and the other surface layer.
(10) With respect to the constitution (9), one surface layer and the other surface layer are formed of high expansion layers and one intermediate layer is formed of a low expansion layer.
(11) With respect to the constitution (9), one surface layer and the other surface layer are formed of low expansion layers and one intermediate layer is formed of a high expansion layer.
(12) With respect to any one of preceding constitutions (8) to (11), the plurality of constituent layers differ in the thermal expansion coefficient in the planer direction from each other.
(13) With respect to any one of preceding constitutions (8) to (12), the thermal expansion coefficient of one surface layer in the planer direction is approximately equal to the thermal expansion coefficient of the other surface layer in the planer direction.
(14) With respect to any one of preceding constitutions (8) to (13), the alloy element is nickel.
Then, with respect to a color cathode ray tube, following constitutions are considered.
(15) In a color cathode ray tube having an evacuated envelope which includes a panel which is coated with phosphor of a plurality of colors on an inner surface thereof, a neck which houses an electron gun and a funnel which connects the panel and the neck and a shadow mask which is installed close to the phosphor coated on the inner surface of the panel and has a large number of color selection apertures, wherein
the shadow mask is constituted of an iron alloy plate which contains iron as a major component and also contains an alloy element,
the concentration of the alloy element is continuously changed in the plate thickness direction of the iron alloy plate, and
the thermal expansion of the iron alloy plate is set lower at an intermediate portion than at respective surface portions in the plate thickness direction of the iron alloy plate.
(16) In a color cathode ray tube having an evacuated envelope which includes a panel which is coated with phosphor of a plurality of colors on an inner surface thereof, a neck which houses an electron gun and a funnel which connects the panel and the neck and a shadow mask which is installed close to the phosphor coated on the inner surface of the panel and has a large number of color selection apertures, wherein
the shadow mask is constituted of an iron alloy plate which contains iron as a major component and also contains an alloy element, and the concentration of the alloy element is continuously changed in the planer direction of the iron alloy plate.
Due to the above-mentioned respective constitutions, following advantageous effects can be obtained.
(a) It is possible to reduce an amount of expensive alloy element such as cobalt, nickel or the like as the material of the shadow mask or it is possible to make such an alloy element unnecessary as the material of the shadow mask so that the reduction of cost can be realized compared to the conventional Invar material. (b) The etching characteristics for forming color selection apertures can be enhanced. (c) The strength of material of the shadow mask can be largely increased (5 to 10 times) compared to the shadow mask which is wholly made of Invar material.
Further, (d) the magnetic shield effect is enhanced due to the enhancement of the magnetic characteristics. (e) Due to the physical structures of three or more layers formed of the composite gradient structure of an alloy element, the partial doming can be reduced. (f) The tolerance for the ambient temperature is enhanced.
Further, by adjusting the thicknesses of respective layers of the plural-layer constitution formed of three or more layers, the partial thermal deformation of the apertured region can be suppressed or the doming generated by the thermal expansion of the whole shadow mask is compensated by the thermal deformation of the skirt portions and hence, the design tolerance of the shadow mask structural body including a suspension mechanism engaged with the inner wall of the panel can be enhanced.
Further, as the alloy element of the iron alloy which constitutes the above-mentioned shadow mask material, chromium or nickel-chromium or the like can be used besides nickel.
It is needless to say that the present invention is not limited to the above-mentioned respective constitutions and structures which will be explained in conjunction with embodiments described later and various modifications are conceivable without departing from the technical concept of the present invention.